ActuallyScar

Sorry to break it to you but Zach has mentioned in a video (at 19:20) that he hasn't even solved the last level of Spacechem and I doubt that has changed since. He has work to do and he's had enough by the end, you know, not going to maximize every level's score.

I'd say probably nodes instead.

Since Zach Barth is a chill guy, he didn't include the in-depth explanation on that (I don't remember where he tells you how he made puzzles).

Now that you have some experience, are multi-optimization challenges a high-priority when making a design-based puzzle game? Or is it just a correlated by-product and it simply happens so that every game has multiple statistics that translate well to optimization challenges after you have already made the bulk of the game?